In an engine control system, a turbocharger can be employed on an internal combustion engine in order to significantly boost the engines horsepower without significantly increasing its weight. Some diesel engines are equipped with two turbochargers, one which operates continuously, and one which is used when there is a demand for high power from the engine. The turbocharger works by engine exhaust flow acting on a turbine, causing it spin. The turbine in charge is connected to a compressor which compresses engine intake air (“boost”) and increases the power of the engine. A butterfly valve, disposed in the engine exhaust passageway, controls exhaust flow to the turbochargers. The valve is closed when the second turbocharger is not needed, thus preventing exhaust flow from entering the second turbocharger. As demand increases, the butterfly valve is opened, allowing exhaust flow to spin the turbine of the second turbocharger, hence providing additional boost.
The butterfly valve may be operated by a servomechanism which includes a three way hydraulic solenoid valve having a spool to control pressure on two sides of a piston. Applying power to the solenoid generates a force on the armature of the solenoid, which in turn moves the spool. As the spool moves, it opens one port and closes another, to apply pressure to one side of the actuator piston, and drain oil from the opposite side of the actuator piston. A force representing the position of the control valve is fed back to the spool by means of a cam and spring to oppose the force applied by the solenoid. When the feedback force and force applied by the solenoid are balanced, the spool valve is brought to the “null” state in which the spool is centered between the ports, and no oil flows, except for leakage flow. As a result, the butterfly valve stops rotating and is held in a desired operating position.
The aforementioned servomechanism has several drawbacks including the high cost of manufacturing the servomechanism.
The embodiments disclosed herein therefore directly address the shortcomings of present servomechanisms by providing a low cost and power efficient hydraulic assembly for controlling an engine exhaust flow control valve.